Cell-to-Cell Signaling: Hormones and Receptors

Read Also

No cell lives in isolation. In all multicellular organisms, survival depends on an elaborate intercellular communication network that coordinates the growth, differentiation, and metabolism of the multitude of cells in diverse tissues and organs. Cells within small groups often communicate by direct cell-cell contact. Specialized junctions in the plasma membranes of adjacent cells permit them to exchange small molecules and to coordinate metabolic responses; other junctions between adjacent cells determine the shape and rigidity of many tissues.

In addition, the establishment of specific cell-cell interactions between different types of cells is a necessary step in the development of many tissues. In some cases a particular protein on one cell binds to a receptor protein on the surface of an adjacent target cell, triggering its differentiation.

How cells communicate by means of extracellular signaling molecules. These substances are synthesized and released by signaling cells and produce a specific response only in target cells that have receptors for the signaling molecules. An enormous variety of chemicals, including small molecules (e.g., amino acid derivatives, acetylcholine), peptides, and proteins, are used in this type of cell-to-cell communication. The extracellular products synthesized by signaling cells can diffuse away or be transported in the blood, thus providing a means for cells to communicate over longer distances than is possible by chains of direct cell-cell contacts.

Overview of Extracellular Signaling

Communication by extracellular signals usually involves six steps: (1) synthesis and (2) release of the signaling molecule by the signaling cell; (3) transport of the signal to the target cell; (4) detection of the signal by a specific receptor protein; (5) a change in cellular metabolism, function, or development triggered by the receptor-signal complex; and (6) removal of the signal, which often terminates the cellular response.

In many eukaryotic microorganisms (e.g., yeast, slime molds, and protozoans), secreted molecules coordinate the aggregation of free-living cells for sexual mating or differentiation under certain environmental conditions. Chemicals released by one organism that can alter the behavior or gene expression of other organisms of the same species are called pheromones. Yeast mating-type factors are a well-understood example of pheromone-mediated cell-to-cell signaling. Some algae and animals also release pheromones, usually dispersing them into the air or water, to attract members of the opposite sex. More important in plants and animals are extracellular signaling molecules that function within an organism to control metabolic processes within cells, the growth of tissues, the synthesis and secretion of proteins, and the composition of intracellular and extracellular fluids.

Signaling Molecules Operate over Various Distances in Animals

In animals, signaling by extracellular, secreted molecules can be classified into three types endocrine, paracrine, or autocrine based on the distance over which the signal acts. In addition, certain membrane-bound proteins on one cell can directly signal an adjacent cell (Figure 1).

In endocrine signaling, signaling molecules, called hormones, act on target cells distant from their site of synthesis by cells of endocrine organs. In animals, an endocrine hormone usually is carried by the blood from its site of release to its target.

In paracrine signaling, the signaling molecules released by a cell only affect target cells in close proximity to it. The conduction of an electric impulse from one nerve cell to another or from a nerve cell to a muscle cell (inducing or inhibiting muscle contraction) occurs via paracrine signaling. The role of this type of signaling, is mediated by neurotransmitters, in transmitting nerve impulses. Many signaling molecules regulating development in multicellular organisms also act at short range.

In autocrine signaling, cells respond to substances that they themselves release. Many growth factors act in this fashion, and cultured cells often secrete growth factors that stimulate their own growth and proliferation. This type of signaling is particularly common in tumor cells, many of which overproduce and release growth factors that stimulate inappropriate, unregulated proliferation of themselves as well as adjacent nontumor cells; this process may lead to formation of tumor mass.

Some compounds can act in two or even three types of cell-to-cell signaling. Certain small amino acid derivatives, such as epinephrine, function both as neurotransmitters (paracrine signaling) and as systemic hormones (endocrine signaling). Some protein hormones, such as epidermal growth factor (EGF), are synthesized as the exoplasmic part of a plasma- membrane protein; membrane-bound EGF can bind to and signal an adjacent cell by direct contact. Cleavage by a protease releases secreted EGF, which acts as an endocrine signal on distant cells.

Receptor Proteins Exhibit Ligand-Binding and Effector Specificity

The cellular response to a particular extracellular signaling molecule depends on its binding to a specific receptor protein located on the surface of a target cell or in its nucleus or cytosol. The signaling molecule (a hormone, pheromone, or neurotransmitter) acts as a ligand, which binds to, or "fits," a site on the receptor. Binding of a ligand to its receptor causes a conformational change in the receptor that initiates a sequence of reactions leading to a specific cellular response.

The response of a cell or tissue to specific hormones is dictated by the particular hormone receptors it possesses and by the intracellular reactions initiated by the binding of any one hormone to its receptor. Different cell types may have different sets of receptors for the same ligand, each of which induces a different response. Or the same receptor may occur on various cell types, and binding of the same ligand may trigger a different response in each type of cell. Clearly, different cells respond in a variety of ways to the same ligand. For instance, acetylcholine receptors are found on the surface of striated muscle cells, heart muscle cells, and pancreatic acinar cells. Release of acetylcholine from a neuron adjacent to a striated muscle cell triggers contraction, whereas release adjacent to a heart muscle slows the rate of contraction. Release adjacent to a pancreatic acinar cell triggers exocytosis of secretory granules that contain digestive enzymes. On the other hand, different receptor-ligand complexes can induce the same cellular response in some cell types. In liver cells, for example, the binding of either glucagon to its receptors or of epinephrine to its receptors can induce degradation of glycogen and release of glucose into the blood.

 These examples show that a receptor protein is characterized by binding specificity for a particular ligand, and the resulting hormone-ligand complex exhibits effector specificity (i.e., mediates a specific cellular response). For instance, activation of either epinephrine or glucagon receptors on liver cells by binding of their respective ligands induces synthesis of cyclic AMP (cAMP), one of several intracellular signaling molecules, termed second messengers, which regulate various metabolic functions; as a result, the effects of both receptors on liver-cell metabolism are the same. Thus, the binding specificity of epinephrine and glucagon receptors differ, but their effector specificity is identical.

In most receptor-ligand systems, the ligand appears to have no function except to bind to the receptor. The ligand is not metabolized to useful products, is not an intermediate in any cellular activity, and has no enzymatic properties. The only function of the ligand appears to be to change the properties of the receptor, which then signals to the cell that a specific product is present in the environment. Target cells often modify or degrade the ligand and, in so doing, can modify or terminate their response or the response of neighboring cells to the signal.

Hormones Can Be Classified Based on Their Solubility and Receptor Location

Most hormones fall into three broad categories: (1) small lipophilic molecules that diffuse across the plasma membrane and interact with intracellular receptors; and (2) hydrophilic or (3) lipophilic molecules that bind to cell-surface receptors (Figure 2). Recently, nitric oxide, a gas, has been shown to be a key regulator controlling many cellular responses.

Lipophilic Hormones with Intracellular Receptors

Many lipid-soluble hormones diffuse across the plasma membrane and interact with receptors in the cytosol or nucleus. The resulting hormone-receptor complexes bind to transcription-control regions in DNA thereby affecting expression of specific genes. Hormones of this type include the steroids (e.g., cortisol, progesterone, estradiol, and testosterone), thyroxine, and retinoic acid All steroids are synthesized from cholesterol and have similar chemical skeletons. After crossing the plasma membrane, steroid hormones interact with intracellular receptors, forming complexes that can increase or decrease transcription of specific genes. These receptor-steroid complexes also may affect the stability of specific mRNAs. Steroids are effective for hours or days and often influence the growth and differentiation of specific tissues. For example, estrogen and progesterone, the female sex hormones, stimulate the production of egg-white hormones in chickens and cell proliferation in the hen oviduct. In mammals, estrogens stimulate growth of the uterine wall in preparation for embryo implantation. In insects and crustaceans, -ecdysone (which is chemically related to steroids) triggers the differentiation and maturation of larvae; like estrogens, it induces the expression of specific gene products.

Thyroxine (tetraiodothyronine) and triiodothyronine the principal iodinated compounds in the body are formed in the thyroid by intracellular proteolysis of the iodinated protein thyroglobulin and immediately released into the blood.

These two thyroid hormones stimulate increased expression of many cytosolic enzymes (e.g., liver hexokinase) that catalyze the catabolism of glucose, fats, and proteins and of mitochondrial enzymes that catalyze oxidative phosphorylation.

Retinoids are polyisoprenoid lipids derived from retinol (vitamin A). They perform multiple regulatory functions in diverse cellular processes. Retinoids regulate cellular proliferation, differentiation, and death, and they have numerous clinical applications. Their diverse effects reflect, at least in part, the multiplicity of retinoid derivatives, the existence of two different classes of receptors that form heterodimers, and differences in their cis-acting regulatory sites on DNA. During development retinoids act as local mediators of cell-cell interaction. For instance, during the formation of motor neurons in the chick, one class of motor neurons generates a retinoid signal which regulates the number and type of neighboring motoneurons.

Water-Soluble Hormones with Cell-Surface Receptors

Because water-soluble signaling molecules cannot diffuse across the plasma membrane, they all bind to cell-surface receptors. This large class of compounds is composed of two groups: (1) peptide hormones, such as insulin, growth factors, and glucagon, which range in size from a few amino acids to protein-size compounds, and (2) small charged molecules, such as epinephrine and histamine (Figure 3), that are derived from amino acids and function as hormones and neurotransmitters.

Many water-soluble hormones induce a modification in the activity of one or more enzymes already present in the target cell. In this case, the effects of the surface-bound hormone usually are nearly immediate, but persist for a short period only. These signals also can give rise to changes in gene expression that may persist for hours or days. In yet other cases water-soluble signals may lead to irreversible changes, such as cellular differentiation.

Lipophilic Hormones with Cell-Surface Receptors

The primary lipid-soluble hormones that bind to cell-surface receptors are the prostaglandins. There are at least 16 different prostaglandins in nine different chemical classes, designated PGA PGI. Prostaglandins are part of an even larger family of 20 carboncontaining hormones called eicosanoid hormones. In addition to prostaglandins, they include prostacyclins, thromboxanes, and leukotrienes. Eicosonoid hormones are synthesized from a common precursor, arachidonic acid. Arachidonic acid is generated from phospholipids and diacylglycerol.

In both vertebrates and invertebrates, prostaglandins are synthesized and secreted continuously by many types of cells and rapidly broken down by enzymes in body fluids.

Many prostaglandins act as local mediators during paracrine and autocrine signaling and are destroyed near the site of their synthesis. They modulate the responses of other hormones and can have profound effects on many cellular processes. Certain prostaglandins cause blood platelets to aggregate and adhere to the walls of blood vessels. Because platelets play a key role in clotting blood and plugging leaks in blood vessels, these prostaglandins can affect the course of vascular disease and wound healing; aspirin inhibits their synthesis by acetylating (and thereby irreversibly inhibiting) prostaglandin H₂ synthase. Other prostaglandins initiate the contraction of smooth muscle cells; they accumulate in the uterus at the time of childbirth and appear to be important in inducing uterine contraction.

Recent studies have shown that a family of plant steroids, called brassinosteroids, regulates many aspects of development. These lipophilic compounds, like prostaglandins, act through cell-surface receptors.

Cell-Surface Receptors Belong to Four Major Classes

The different types of cell-surface receptors that interact with water-soluble ligands are schematically represented in Figure 4. Binding of ligand to some of these receptors induces second-messenger formation, whereas ligand binding to others does not. For convenience, we can sort these receptors into four classes:

• G protein coupled receptors (Figure 4a): Ligand binding activates a G protein, which in turn activates or inhibits an enzyme that generates a specific second messenger or modulates an ion channel, causing a change in membrane potential. The receptors for epinephrine, serotonin, and glucagon are examples.
• Ion-channel receptors (Figure 4b): Ligand binding changes the conformation of the receptor so that specific ions flow through it; the resultant ion movements alter the electric potential across the cell membrane. The acetylcholine receptor at the nerve-muscle junction is an example.
• Tyrosine kinase linked receptors (Figure 4c): These receptors lack intrinsic catalytic activity, but ligand binding stimulates formation of a dimeric receptor, which then interacts with and activates one or more cytosolic protein-tyrosine kinases. The receptors for many cytokines, the interferons, and human growth factor are of this type. These tyrosine kinase linked receptors sometimes are referred to as the cytokine-receptor superfamily.
• Receptors with intrinsic enzymatic activity (Figure 4d): Several types of receptors have intrinsic catalytic activity, which is activated by binding of ligand. For instance, some activated receptors catalyze conversion of GTP to cGMP; others act as protein phosphatases, removing phosphate groups from phosphotyrosine residues in substrate proteins, thereby modifying their activity. The receptors for insulin and many growth factors are ligand-triggered protein kinases; in most cases, the ligand binds as a dimer, leading to dimerization of the receptor and activation of its kinase activity. These receptors often referred to as receptor serine/threonine kinases or receptor tyrosine kinases autophosphorylate residues in their own cytosolic domain and also can phosphorylate various substrate proteins.

Effects of Many Hormones Are Mediated by Second Messengers

The binding of ligands to many cell-surface receptors leads to a short-lived increase (or decrease) in the concentration of the intracellular signaling molecules termed second messengers. These low-molecular-weight signaling molecules include 3,5-cyclic AMP (cAMP); 3,5-cyclic GMP (cGMP); 1,2-diacylglycerol (DAG); inositol 1,4,5-trisphosphate (IP₃); various inositol phospholipids (phosphoinositides); and Ca²⁺ (Figure 5, 6).

The elevated intracellular concentration of one or more second messengers following hormone binding triggers a rapid alteration in the activity of one or more enzymes or nonenzymatic proteins. The metabolic functions controlled by hormone-induced second messengers include uptake and utilization of glucose, storage and mobilization of fat, and secretion of cellular products. These intracellular molecules also control proliferation, differentiation, and survival of cells, in part by regulating the transcription of specific genes. The mode of action of cAMP and other second messengers is discussed in a later section. Removal (or degradation) of a ligand or second messenger, or inactivation of the ligand-binding receptor, can terminate the cellular response to an extracellular signal.

Other Conserved Proteins Function in Signal Transduction

In addition to cell-surface receptors and second messengers, several types of conserved proteins function in signal transduction pathways stimulated by extracellular signals. Here we introduce the three main classes of these intracellular signaling proteins; their structures and functions are described in detail in later sections.

GTPase Switch Proteins

A large group of GTP-binding proteins act as molecular switches in signal-transduction pathways. These proteins are turned "on" when bound to GTP and turned "off" when bound to GDP (Figure 7). In the absence of a signal, the protein is bound to GDP. Signals activate the release of GDP, and the subsequent binding to GTP over GDP is favored by the higher concentrations of GTP in the cell. The intrinsic GTPase activity of these GTP-binding proteins hydrolyzes the bound GTP to GDP and P_i, thus converting the active form back to the inactive form. The kinetics of hydrolysis regulates the length of time the switch is "on."

There are two classes of GTPase switch proteins: trimeric G proteins, which as noted already are directly coupled to certain receptors, and monomeric Ras and Ras-like proteins. Both classes contain regions that promote the activity of specific effector proteins by direct protein-protein interactions. These regions are in their active conformation only when the switch protein is bound to GTP. G proteins are coupled directly to activated receptors, whereas Ras is linked only indirectly via other proteins. The two classes of GTPbinding proteins also are regulated in very different ways.

Protein Kinases

Activation of all cell-surface receptors leads to changes in protein phosphorylation through the activation of protein kinases (Figure 7b). In some cases kinases are part of the receptor itself, and in others they are found in the cytosol or associated with the plasma membrane. Animal cells contain two types of protein kinases: those directed toward tyrosine and those directed toward either serine or threonine. The structures of the catalytic core of both types are very similar. In general, protein kinases become active in response to the stimulation of signaling pathways. The catalytic activities of kinases are modulated by phosphorylation, by direct binding to other proteins, and by changes in the levels of various second messengers. The activity of protein kinases is opposed by the activity of protein phosphatases, which remove phosphate groups from specific substrate proteins..

Adapter Proteins

Many signal-transduction pathways contain large multiprotein signaling complexes, which often are held together by adapter proteins (Figure 7c). Adapter proteins do not have catalytic activity, nor do they directly activate effector proteins. Rather, they contain different combinations of domains, which function as docking sites for other proteins. For instance, different domains bind to phosphotyrosine residues (SH2 and PTB domains), proline-rich sequences (SH3 and WW domains), phosphoinositides (PH domains), and unique C-terminal sequences with a C-terminal hydrophobic residue (PDZ domains). In some cases adapter proteins contain arrays of a single binding domain or different combinations of domains. In addition, these binding domains can be found alone or in various combinations in proteins containing catalytic domains. These combinations provide enormous potential for complex interplay and cross-talk between different signaling pathways.

 Common Signaling Pathways Are Initiated by Different Receptors in a Class

In general, different members of a particular class of receptors transduce signals by highly conserved pathways. Moreover, analogies are found in the signaling pathways associated with different receptor classes. Figure 8 illustrates the main components of the key signaling pathways downstream from G proteincoupled receptors (GPCRs) and receptor tyrosine kinases (RTKs), the two receptor classes that we consider in detail in this chapter. Although a GTPase switch protein occurs in both types of pathways, its position in the pathway relative to the receptor differs. Second messengers are critical components of most GPCR pathways and some RTK pathways. Adapter proteins function in all RTK pathways but not in the main GPCR pathways. Protein kinases, however, play a key role in all signaling pathways; ultimately an activated protein kinase phosphorylates one or more substrate proteins. The nature of the substrate proteins, which include enzymes, microtubules, histones, and transcription factors, plays an important role in determining the cellular response to a particular signal in a particular cell.

The Synthesis, Release, and Degradation of Hormones Are Regulated

Because of their potent effects, hormones and neurotransmitters must be carefully regulated. The release and degradation of some signaling compounds are regulated to produce rapid, short-term effects; others to produce slower-acting but longer-lasting effects (Table 1). In some cases, complex regulatory networks coordinate the levels of hormones whose effects are interconnected.

Peptide Hormones and Catecholamines

Organisms must be able to respond instantly to many changes in their internal or external environment. Such rapid responses are mediated primarily by peptide hormones and the catecholamines epinephrine, norepinephrine, and dopamine (see Figure 3). The cells that produce these signaling molecules store them in secretory vesicles just under the plasma membrane. The supply of stored, preformed signaling molecules is sufficient for 1 day in the case of peptide hormones and for several days in the case of catecholamines. All peptide hormones, including insulin and adrenocorticotropic hormone (ACTH), are synthesized as part of a longer propolypeptide, which is cleaved by specific proteases to generate the active molecule just after it is transported to a secretory vesicle (see Figure 10).

Stimulation of signaling cells causes immediate exocytosis of the stored peptide hormone or catecholamine into the surrounding medium or the blood. Secreting cells also are stimulated to synthesize the signaling molecule and replenish the cell's supply. Released peptide hormones persist in the blood for only seconds or minutes before being degraded by blood and tissue proteases. Released catecholamines are rapidly inactivated by different enzymes or taken up by specific cells. The initial actions of these signaling molecules on target cells (the activation or inhibition of specific enzymes) also last only seconds or minutes. Thus the catecholamines and some peptide hormones can mediate short responses that are terminated by their own degradation.

Steroid Hormones, Thyroxine, and Retinoic Acid

The pathways for synthesizing steroid hormones from cholesterol involve 10 or more enzymes. Steroid-producing cells, like those in the adrenal cortex, store a small supply of hormone precursor but none of the mature, active hormone. When stimulated, the cells convert the precursor to the active hormone, which then diffuses across the plasma membrane into the blood. Likewise, thyroglobulin, the iodinated precursor of thyroxine is stored in thyroid follicles. When cells lining these follicles are exposed to thyroid-stimulating hormone (TSH), they take up thyroglobulin; controlled proteolysis of this glycoprotein by lysosomal enzymes yields thyroxine, which is released into the blood.

Because the signaling cells that produce thyroxine and steroid hormones store little of the active hormone, release of these hormones takes from hours to days (see Table 1). These hormones, which are poorly soluble in aqueous solution, are transported in the blood by carrier proteins; the tightly bound active hormones are not rapidly degraded. Thus, cellular responses to thyroxine and steroid hormones take awhile to occur but persist from hours to days.

Retinol is stored in the liver and is found in high concentrations in blood in a complex with serum binding protein. Due to its lipophilic nature, retinol diffuses through the plasma membrane and forms a complex with a cytosolic retinol-binding protein called CRBP. Retinol is converted to retinal through the activity of retinol dehydrogenase, and retinal, in turn, is converted to retinoic acid by retinal dehydrogenase. Retinoic acid can act as a signal in the cell in which it is produced, or it can diffuse through the plasma membrane to influence the development of neighboring cells. Retinoic acid can also be further modified enzymatically to alter its signaling specificity.

Feedback Control of Hormone Levels

The synthesis and/or release of many hormones are regulated by positive or negative feedback. This type of regulation is particularly important in coordinating the action of multiple hormones on various cell types during growth and differentiation. Often, the levels of several hormones are interconnected by feedback circuits, in which changes in the level of one hormone affect the levels of other hormones. One example is the regulation of estrogen and progesterone, steroid hormones that stimulate the growth and differentiation of cells in the endometrium, the tissue lining the interior of the uterus. Changes in the endometrium prepare the organ to receive and nourish an embryo. The levels of both hormones are regulated by complex feedback circuits involving several other hormones.